1. Field of the Invention
The present invention relates to a transistor circuit such as a transistor-transistor-logic (TTL) circuit, more particularly to a pull-down circuit for an output transistor in the TTL circuit.
2. Description of the Prior Art
A typical TTL circuit is comprised of a main inverting amplifier composed of transistors, resistors, and a pulldown circuit. The main inverting amplifier receives an input signal IN and produces an inverted phase signal as an output signal OUT. The pull-down circuit functions to discharge electric charges stored at a base of one of the abovementioned transistors, that is, an output transistor, when the output transistor is turned from on to off. This shortens the turn-off time. Accordingly, the level of the output signal OUT quickly rises from "L" (low) to "H" (high).
However, there is a problem with the above-mentioned main inverting amplifier. The problem is that the output signal cannot change stepwise from "L" to "H", and vice versa, but changes with a gradual slope. The reason for this will be explained hereinafter. In any case, this results in a reduced "noise margin" for the operation of the TTL circuit.